a) Field of the Invention
This invention is directed to an arrangement for optical surface profile measurement and for obtaining optical sectional images of transparent, partially transparent and opaque objects. An arrangement of this kind can be used in many areas. Currently, an arrangement of this kind appears to have considerable value above all in medicine, but is not limited thereto.
b) Description of the Related Art
Two fundamental optical-tomographic methods were developed in the nineteen-nineties. In optical scattered light tomography, absorption coefficients and scattering coefficients of the object area to be imaged are determined from the characteristic influence of light beams which pass through the object area to be imaged such that they are repeatedly scattered. This method provides images with poor spatial resolution. The second method, optical coherence tomography (generally referred to as OCT), is based on coherence characteristics of light and delivers high-resolution images (D. Huang; E. A. Swanson; C. P. Lin; J. S. Schuman; W. G. Stinson; W. Chang; M. R. Hee; T. Flotte; K. Gregory; C. A. Puliafito; J. G. Fujimoto: Optical coherence tomography, Science, 254 (1991), pages 1178-1181).
Actually, OCT should really be called xe2x80x9coptical short-coherence tomographyxe2x80x9d because it involves the use of short coherent light, that is, light with a distinct spectral width and therefore short time coherence. In OCT, the object is scanned point by point along a line extending on the object surface in the x-direction by the measurement beam of an interferometer. Under every surface point the measurement beam also penetrates into the object (in the z-direction) and the diffusely reflected light is interfered with the reference beam of the interferometer. Interference occurs because of the use of short coherence light only when the measurement beam and reference beam have the same path length within the coherence length. Through continuous traversing of the measurement distance with the reference mirror, the depth position z and light scattering intensity of the locations diffusely reflecting the light are recorded under every surface point in the measured object. This can also be referred to as an xe2x80x9coptical A-scanxe2x80x9d in analogy to the more familiar ultrasound method (it differs from ultrasound, however, in that the ultrasound A-scan gives the depth position of the reflection locations of sound echoes based on the transit time from the interior of the object, whereas in OCT the depth position is determined by way of the path length balance between the measurement beam and reference beam which is required for interference.) Finally, the OCT tomogram is compiled line by line from many optical A-scans which are offset relative to one another in the x-direction.
In the meantime, OCT has become a successful diagnostic method, especially in ophthalmology. However, a disadvantage in the original OCT process consists in that the reference mirror must be moved mechanically. This is a source of wear and limits the image acquisition speed.
An alternative to the optical A-scan described above is the spectral interferometric method. In this method, the depth position z of the object locations which diffusely reflect the light is determined from the wavelength spectrum of the diffusely reflected light as is described in L. M. Smith and C. C. Dobson, Applied Optics, 1989, vol. 28, no. 15, pages 3339-3342. This process also forms the basis for the Austrian Patent Application A216/93-1 and the German Application DE 43 09 056 A1. The optical A-scan is obtained in this case by a Fourier transform of the spectral intensity distribution of the light diffusely reflected by the object.
In this last method described above, the spectrum of the light diffusely reflected by the object is obtained, for example, by a diode array in the spectrometer plane. However, direct application of the Fourier transform does not give the actual A-scan, but rather the autocorrelation function of the A-scan [the strict reasons for this are given, for example, in A. F. Fercher, C. K. Hitzenberger, G. Kamp, S. Y. El-Zaiat: Measurement of Intraocular Distances by BackscatteringSpectral Interferometry, Opt. Commun. 117 (1995) 43-48]. The physical basis for his consists in the fact that the diode array records the light diffusely reflected by he object only with respect to its wavelength-dependent intensity (or the square root thereof, which is the amplitude), but the phase is expressly ignored. In order to gain access to the phase as well, a reference wave with a path difference relative to the object that exceeds the object depth is required. However, this leads to a considerable increase in the spatial frequencies in the spectrum which must also be resolved by the diode array. Thus, in this case, a part of the available resolution capacity of the diode array must be used simply to register the phase without increasing the resolution of the imaging. However, since the resolution of this OCT process is determined by the resolution of the available diode array, this causes an unnecessary limitation of the resolution of the OCT images.
In contrast, the arrangement according to the invention also uses a reference wave, but in such a way that no additional demands are placed on the resolution capacity of the diode array.
The arrangement according to the invention is based on a process described in the literature (in: Fercher, A. F.; Hitzenberger, C. K.; Kamp, G.; El-Zaiat, S. Y.: Measurement of Intraocular Distances by Backscattering Spectral Interferometry, Opt. Commun. 117 (1995) 43-48) according to which the distribution of the light-scattering locations in the z-direction along the measurement beamxe2x80x94or, more strictly speaking, the scattering potential F(z)xe2x80x94is obtained through Fourier transformation of the complex spectral amplitude, that is, from spectral amplitude amount A(xcex) and spectral phase xcfx86(xcex) of the diffusely reflected light beam. That is, the amplitude and phase must be measured in the diffusely reflected light for the used wavelengths. For this purpose, a reference beam is also used in the process according to the invention; however, this reference beam requires no additional path difference relative to the object light. The reference beam is changed in phase by discrete steps and the amplitude and phase of the object light are determined from the associated spectral intensities in the interferogram. Phase measurement methods of this type are described in prior art digital interferometry and, for example, in the textbook xe2x80x9cOptical Interferometryxe2x80x9d, P. Hariharan, Academic Press 1985, ISBN 0 12 325220 2.
A primary object of the present invention, then, is to overcome the disadvantages of the OCT method which uses a reference wave, but which places no additional demands on the resolution capacity of the diode array.
Therefore, the arrangement according to the invention substantially comprises an illumination beam which illuminates the measurement object, a spectrometer grating or a spectrometer prism or prisms which spectrally separate the object light diffusely reflected by the measurement object, a reference beam which can be varied by steps with respect to phase relative to the object beam and which is superposed on the object beam, and a detector array which measures the spectral intensities of the interferogram in the different phase values of the reference light. This will be described more fully in the following with reference to the drawings.